Anthrax is an infection caused by a bacterium, Bacillus anthracis. The infection can take three forms depending on the affected part of the body: cutaneous (on the skin), inhalational, and gastrointestinal. Bacillus anthracis is a microorganism that lives in the soil as a spore. It is hardy and can lie dormant yet alive for many years. The bacterium primarily infects wild and domesticated grazing animals, such as cattle, deer, sheep, and goats. Humans can then become infected by handling the animals or their hair, hide, or meat.

Natural cases of human anthrax infection are rare in the United States. For the 45 years from 1955 through 1999, there were 236 reported cases of anthrax, and 224 of them were cutaneous.

Anthrax received substantial attention in 2001 when multiple people were exposed to anthrax spores sent through the U.S. mail. Since then, procedures have been implemented to monitor hundreds of key postal distribution centers for bioterrorism agents such as anthrax in order to protect postal customers and employees.

Another high profile case involving anthrax occurred in 2006 when a New York City man was exposed while using imported goat hides to make traditional African drums.

The cases in 2001 are believed to have been intentionally caused and raised concerns about the use of anthrax as a weapon. Many countries have investigated and experimented with anthrax as a biological weapon. It can be a potentially effective weapon for terrorists because of its characteristics. Though anthrax is not spread from person to person, the B. anthracis spores are hardy enough to survive dissemination through various routes and infections can be fatal.

Bacillus anthracis infects and produces toxins that can damage cells directly and cause inflammation and swlling (edema). The three kinds of infection vary in their symptoms and prognoses:

Cutaneous anthrax is the most common form and is usually caused by the bacterium getting into a cut or abrasion on the skin, as can happen from contact with contaminated meat, wool, hides, or leather. The incubation period is 1 to 12 days. The infection begins as a bump that looks like an insect bite and within days opens into a painless ulcer with a black area in the center. Nearby lymph glands may swell. About 20% of untreated cases result in death, but death is rare in those who receive antimicrobial therapy.

Inhalational anthrax is the most lethal form and is caused by breathing in spores. The incubation period is believed to be 1 to 7 days generally but may range up to 60 days. There are two stages to the infection. Initial symptoms resemble those of a viral respiratory illness, including sore throat, mild fever, muscle aches, a non-productive cough, and malaise. This first stage can last from hours to a few days and the infected person may temporarily begin to feel somewhat better. The second stage may develop suddenly, with signs and symptoms including shortness of breath, high fever, shock, meningitis, chest pain, and respiratory failure. The fatality rate is high, about 75% after symptoms begin, even with medical care, including antibiotics.

Gastrointestinal anthrax, caused by the consumption of raw or undercooked contaminated meat, has an incubation period of 1 to 7 days. The symptoms include severe abdominal distress and fever. The symptoms can be concentrated around either the pharynx, with lesions at the base of the tongue, sore throat, fever, and enlarged lymph nodes, or the lower bowels, with nausea, loss of appetite, vomiting, and fever, followed by abdominal pain, vomiting blood, and bloody diarrhea. The fatality rate is 25% to 60%.

Anthrax infection is diagnosed by culturing the bacteria, using a specimen appropriate to the form of the disease suspected, such as from blood, skin lesions, or respiratory secretions, or by measuring antibodies in the blood. For inhalational anthrax, a chest X-ray can also be helpful as can a test of cerebrospinal fluid if signs of meningitis are present.

Culturing of a sample from a bodily fluid such as blood can take several hours to several days. The specimen is incubated in artificial media, where the bacteria can grow. (Read more about blood culture.) Conventional biochemical tests and molecular tests are then performed to identify the bacteria and susceptibility testing is done to select the best antibiotic for therapy.

Testing may also be performed to rapidly detect anthrax DNA in the blood and to confirm culture findings. Because of the incident in 2001, there is great interest in being able to rapidly detect anthrax exposure and infection. Other tests are being developed with this in mind.

There is a protective vaccine for anthrax, but it is recommended for use only in high-risk populations, such as military personnel, some veterinarians, and people who work with Bacillus anthracis in laboratories. It is not available to the general public.

There are recommendations for giving a combination of antimicrobial drugs to people who have been exposed or thought to have been exposed to Bacillus anthracis and to people who have been diagnosed with inhalational anthrax. It is important that medical intervention begin soon after diagnosis. If a person has meningitis, a third antimicrobial may also be given. In some cases, an antitoxin will also be prescribed. Typical treatment courses are for 60 days. For the first couple of weeks, the drugs will usually be given intravenously.

For cutaneous anthrax, antimicrobial treatment is given for about 7-10 days. If there is any indication that the infection was not naturally acquired (from contaminated hides, meat, etc.), then a 60-day course of preventive antimicrobials may be prescribed.

NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

(Updated 2013 August 29). Basics, What is anthrax? Centers for Disease Control and Prevention [On-line information]. Available online at http://www.cdc.gov/anthrax/basics/index.html through http://www.cdc.gov. Accessed June 2014.

(Updated 2013 August 29). Laboratory Testing for Anthrax: Frequently Asked Questions. Centers for Disease Control and Prevention [On-line information]. Available online at http://www.cdc.gov/anthrax/labs/labtestingfaq.html through http://www.cdc.gov. Accessed June 2014.

(Updated 2009 July 17). Anthrax, General Information. CDC, National Center for Zoonotic Vector-Borne, and Enteric Diseases [On-line information]. Available online at http://www.cdc.gov/nczved/divisions/dfbmd/diseases/anthrax/ through http://www.cdc.gov. Accessed June 2010.

(Current as of 2010 April 28). What is Anthrax? United States Department of Labor, Occupational Safety and Health Administration, eTool, [On-line information]. Available online at http://www.osha.gov/SLTC/etools/anthrax/disease_rec.html through http://www.osha.gov. Accessed July 2010.